Routing method

ABSTRACT

In the routing method and apparatus a second routing attempt via an alternate path from an originating node is implemented for a call given an unsuccessful routing attempt via a preferred route, i.e. the call is offered a further alternate path to the destination node. The evaluation of the effectiveness of the rerouting is updated at each call per traffic relationship (originating/destination node pair), using an evaluation storing device. A decision is made based on the evaluation as to whether the rerouting for this traffic relationship is to be maintained or throttled for following calls.

BACKGROUND OF THE INVENTION

When a transient switching center does not find a free line in aconnection setup (for example, as a result of an occupied route), thenthe preceding switching center can be informed of this uponcollaboration of a corresponding backward signaling. When the originalconnection is set up again in the preceding switching center via adifferent route, then this method is referred to as “rerouting” or“crankback”.

Under normal load conditions, the probability that a connection can besuccessively set up through the network is enhanced with this reroutingmethod. Under high load conditions, however, the rerouting method leadsto higher network blockings than given a connection set up withoutrerouting methods. At the same time, the rerouting generates anadditional blind load under high load, which leads to a furtherworsening of the load situation in the network.

Up to now, this problem was solved in that the operator manuallydisconnects in this high-load cases.

This procedure has the following disadvantages:

A plurality of measured traffic data would have to be acquired.

This would not be practical without an involved and expensive trafficmanagement center.

Manual interventions are susceptible to error.

The rerouting would have to be designationally shut off for therespectively affected traffic relationships, which is not practicalmanually in large networks because of the great plurality of trafficrelationships.

SUMMARY OF THE INVENTION

The present invention is based on the object of avoiding the abovedescribed disadvantages.

In general terms the present invention is a routing method. A secondrouting attempt via an alternate path from an originating node isimplemented for a call given an unsuccessful routing attempt via apreferred route, i.e. the call is offered a further alternate path tothe destination node. The evaluation of the effectiveness of thererouting is updated at each call per traffic relationship(originating/destination node pair), using an evaluation storing means.A decision is made on the basis of the evaluation as to whether thererouting for this traffic relationship is to be maintained or throttledfor following calls.

Advantageous developments of the present invention are as follows.

Effectiveness of the rerouting is evaluated in that a respectivererouting account is maintained as evaluation storing means per trafficrelationship (originating/destination node pair). Given a successfulrouting of a call via an initially offered alternate route, the statusof the account allocated to the traffic relationship of this call isincremented by a first amount. Given a rerouting required for a call,the status of the account allocated to the traffic relationship of thiscall is deincremented by a second amount. The rerouting is throttled froa specific traffic relationship when the status of the account belongingto this traffic relationship falls below a specific threshold.

The present invention is also a rerouting system of a switching node ina communication network.

A rerouting means that, following an unsuccessful routing attempt of acall via a preferred route, a second routing attempt via an alternatepath is implemented fro this call via an alternate path. A reroutingmonitoring means that maintains an evaluation storing means per trafficrelationship (originating/destination node pair) using the evaluation ofthe effectiveness of the rerouting is updated at each call, whereby thererouting monitoring means decides on the basis of the updatedevaluation as to whether the rerouting for this traffic relationship isto be maintained or throttled for following calls.

The rerouting monitoring means evaluates the effectiveness of thererouting in that it maintains a respective rerouting account asevaluation storing means per traffic relationship(originating/destination node pair). The rerouting monitoring means,given a successful routing of a call via an initially offered alternateroute, increments the status of the account allocated to the trafficrelationship of this call by a first amount. The rerouting monitoringmeans, given a rerouting required for a call, deincrements the status ofthe account allocated to the traffic relationship of this call by asecond amount. The rerouting monitoring means, throttles the reroutingfor a specific traffic relationship when the status of the accountbelonging to this traffic relationship falls below specific threshold.The rerouting monitoring means throttles in that it temporarilysuppresses the rerouting.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like element s and in which:

FIG. 1 depicts the structure of the routing system according to thepresent invention;

FIG. 2 is a graph depicting the link workload to a destination node;

FIG. 3 is a graph depicting traffic to and from the destination node;

FIG. 4 is a graph depicting the differences in the traffic to thedestination node;

FIG. 5 is a graph depicting rerouting attempts; and

FIG. 6 is a graph depicting rerouting attempts with throttling.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the structure of the inventive routing system. Theinventive system comprises a rerouting means as well as a reroutingmonitoring means. The rerouting monitoring means maintains an evaluationmemory means per traffic relationship (source-destination relationship),for example a bucket or an account (for example, a counter) with whoseassistance an evaluation is made at every call to see whether thererouting remains in force or is disconnected or, respectively,suppressed.

For every call successively routed via an alternate route, the bucket(or, respectively, the account status) is filled (or, respectively,incremented) by a specific quantity (or, respectively, specific amountG). The bucket (or, respectively, the account) exhibits an upper limitfor the filling level (or, respectively, the account status) that isreferred to below as maximum reading Cmax and that is independent of thetraffic relationship.

On the other hand, the bucket or, respectively, the account isdecremented by a specific amount S for every call for which a reroutingmust be implemented. When the bucket is empty or, respectively, thereading falls below a specific threshold, the rerouting is suppressedfor the next call or, respectively, the next calls of the same trafficrelationship. The event that in turn ends the suppression of thererouting for a traffic relationship can, for example, be the expirationof a specific time span. The end of the suppression is achieved by arenewed filling of the appertaining account (for example, to the initialvalue or to a value that lies just above the threshold).

The following data control the throttling of the rerouting:

Semi-Permanent Data of the System Program System:

Cmax: Maximum reading of the bucket, for example Cmax=40, patchable

G: Gain for every successfully routed call on an alternate route, forexample G=1, patchable

S: Loss for every rerouted call, for example S=2, patchable

Transient Data Per Traffic Relationship:

nB: Non-negative integer that indicates the content of the bucket,initialization of the bucket: nB=0.75 Cmax.

Said exemplary values were selected on the basis of a simulation thatconcentrated on the investigation of overload situations and whoseresults are presented with the assistance of FIGS. 2 through 6.

In this simulation, an exemplary, symmetrical, fully interworked networkwith 23 nodes and 120 channels per link was investigated, namely underthe following load conditions:

2 paths basic load=0.75 Erl/channel,

concentrated overload offering of one node 1-22 to the node 23 (one pathoverload) of 0.375 Erl/channel up to 3 Erl/channel (in the example, theapplication of a dynamic routing method was assumed. However, theapplication of the inventive method is also possible and meaningfulwithout limitation given non-dynamic routing (for example, “FixedAlternate Routing”)).

FIG. 2 shows the result of said simulation for the quotient from thenumber of calls for which a rerouting was initiated and the number ofcalls that were successfully routed via alternate routes dependent onthe offered overload (given deactivated throttling). In addition, FIG. 2shows the link workload to the destination node 23 and the blockingprobability for calls wherein rerouting ensued.

As soon as the links to the destination node start to go into saturation(85%-95%), the probability, accordingly, rapidly increases that a callfor which rerouting ensued will be blocked, i.e. the rerouting becomesextremely inefficient. Parallel thereto, said quotient increases by twoorders of magnitude. Said quotient is therefore a very sensitiveparameter for investigating the effect of the inventive reroutingthrottling mechanism.

As soon as said quotient reaches a value of approximately 0.6, theblocking probability for a call rerouted anew increases to more than60%, and the link workload (link utilization) increases to 96%. G=1 andS=2 were selected as parameters in FIG. 1.

In order to check, on the one hand, that the routing performances is notinfluenced by the inventive throttling method and, on the other hand,that the network workload is reduced, the overload scenario of FIG. 2was simulated with the inventive throttling mechanism and without theinventive throttling mechanism.

FIG. 3 shows the carried traffic to and from the destination node 23 forthe case of rerouting with throttling mechanism. It can be seen fromFIG. 3 that the throttling procedure is active when the channelresources to the destination node (22×120=2640 channels) are nearlyused, i.e. the traffic to destination node 23 ensues only over alternatepaths (substitute paths) nearly without exception. After this point, thecarried traffic that terminates at the destination node deviates moreand more from the carried traffic that arises in the destination node.

Since differences (Deltas) compared to that case wherein the reroutingis not throttled would not be visible in FIG. 3 due to its orders ofmagnitude, these Deltas are shown in the carried traffic(Delta=non-throttled minus throttled) in FIG. 4.

FIG. 4 shows the Deltas in the carried traffic to the destination node23 between the unthrottled and throttled case. Before the throttlingexhibits any effect, the differences (Deltas) are of a purelystatistical nature (different random spot checks (patterns) are used).When, however, the offered traffic becomes so high that the throttlingof the rerouting becomes active, then the carried traffic to thedestination node is reduced by less than 1 percent. This reduction islargely compensated by a gain in the carried traffic in the oppositedirection. Overall, the reduction of the carried traffic from and to thedestination node due to the throttling of the rerouting call attempts isthus negligible (less than 0.2%).

FIG. 5 illustrates that without the throttling, the rerouting attemptsstart to flood the network as soon as the trunk resources are used up.These rerouting attempts thus contribute substantially to the callprocessing load generated overall within the network nodes. Sincererouting attempts mainly add to the load of the transient nodes, it caneven occur that a limited network degradation (outage) leads to anetwork-wide overload. As can be seen from FIG. 5 and in a logarithmicscaling in FIG. 6, the throttling is extremely effective and, in theillustrated example, leads to a maximum rerouting attempt rate of lessthan one rerouting attempt per second within the network.

The maximum rate is thereby achieved at what is referred to as thethrottling threshold point. When the offered traffic increases over andabove this, said rate is rapidly reduced, by contrast whereto said rateincreases in unlimited fashion with the offered traffic without thethrottling.

The remaining increase in the processing load (see FIG. 5) is caused bythe call processing in the originating node and in the transient node ofthe first alternative route that is sought. This increase (rise) couldonly be reduced if the alternate routing itself were to be throttled.This throttling of the alternate routing, however, is extremelyproblematical since, as a result thereof, effects negatively influencingthe routing performance far more can occur than due to the throttling ofthe rerouting.

In summary, it can be stated that the results of the simulations thathave been presented show that the disclosed, inventive throttlingmechanism effectively protects the network against overload situationswithout negatively influencing the routing performance.

The invention is not limited to the particular details of the method andapparatus depicted and other modifications and applications arecontemplated. Certain other changes may be made in the above describedmethod and apparatus without departing from the true spirit and scope ofthe invention herein involved. It is intended, therefore, that thesubject matter in the above depiction shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:
 1. A routing method comprising the steps of: causingan originating node to implement rerouting a call given an unsuccessfulrouting attempt via an alternate path first offered according to adirect path or a planned path; maintaining a memory for evaluation ofthe rerouting per traffic relationship (originating/destination nodepair); incrementing by a first amount, the value of the memory allocatedto the traffic relationship of the call given a successful routing ofthe call via an alternate path first offered; deincrementing by a secondamount, the value of the memory means allocated to the call given thererouting required; and disconnecting or suppressing the rerouting for aspecific traffic relationship when the value of the memory belonging tothe traffic relationship falls below a specific threshold.
 2. A routingsystem of a switching node in a communication network, the systemcomprising: a rerouter that given an unsuccessful routing attempt madevia an alternate first offered direct path route or planned path route,offers a call another alternate path; a routing monitor; and a memory,such that a successful routing of a call via the alternate first offeredpath, increments a value in the memory allocated to a trafficrelationship of the call by the first amount, and such that a reroutingrequired for the call the memory deincrements a value in the memoryallocated to the call by a second amount, whereby the memory disconnectsor suppresses rerouting for a specific traffic relationship when a valueof memory belonging to the traffic relationship falls below a specificthreshold.